1. Field of the Invention
The present invention relates to a liquid crystal display device which features high light transmittance and high contrast and, more particularly, to a lightshielding layer thereof.
2. Related Background Art
Information-oriented society is rapidly growing with growing popularity of multimedia. In this trend, a thin, flat display is becoming an important device to expand multimedia market as a replacement of cathode ray tube (CRT) serving as an interface between computer and man. A liquid crystal display, a plasma display (PDP), and an electron beam flat display are promising candidates as such a flat display. Among these promising candidates, liquid crystal displays are finding a significantly expanding market with the increasing popularity of small personal computers. Among the liquid crystal displays, an active matrix liquid crystal display provides a high contrast on a whole screen because it is free of crosstalk in comparison with a direct matrix liquid crystal display such as an STN liquid crystal display. Due to this feature, the active matrix liquid crystal display is attracting attention in such applications as the viewfinder of a video camera, projector, and thin television as well as the display of a small personal computer.
A liquid crystal display includes a liquid crystal material held between two electrodes, the surfaces of which have been subjected to rubbing treatment. Voltage is applied to the electrodes to apply an electric field to the liquid crystal; the orientation of liquid crystal molecules is controlled by the electric field to modulate the polarizing direction of light, thereby turning ON or OFF the light. For the liquid crystal, twisted nematic (TN) liquid crystal is frequently used.
In an active matrix liquid crystal display device, the orientation of the liquid crystal is changed by turning ON or OFF the application of a predetermined electric field which is present between a picture element electrode and an opposing electrode. Most of recent liquid crystal display devices employ normally-white mode wherein light is let pass through to provide white display when no electric field is applied between the two electrodes whereas the light is not allowed to pass through to provide black display when the electric field is applied.
The electric field generated by the picture element electrode and the opposing electrode is nonuniform around the picture element electrode, letting light pass through in the vicinity of the outer periphery of the picture element electrode in the case of the black display, resulting in a deteriorated contrast of the display device. This phenomenon is called disclination. In the past, the vicinity of the outer periphery of the picture element electrode is shielded against light to avoid the contrast degradation by the disclination.
FIG. 4 shows a conventional lightshielding layer. In FIG. 4, reference numeral 501 denotes one of the picture element electrodes and reference numeral 502 denotes an opening of the lightshielding layer. The periphery of the picture element electrode 501 matches with the outer periphery of the lightshielding layer. All areas except the area of 502 are shielded against light, thus always providing the black display regardless of the voltage applied to the picture element electrode. The area where the disclination takes place depends on the direction in which the picture element electrode has been rubbed and therefore, the disclination grows larger upward when the picture element electrode has been rubbed from top to bottom in the direction as shown by arrow E in FIG. 4. For this reason, as shown in the drawing illustrative of the position of the picture element electrode 501 with respect to the opening 502, it is effective to set different distances at the top and bottom of the opening 502 by setting a distance 503 larger than a distance 504, for example (Japanese Patent Laid-Open Application No. 1-266512). In the past, the distances 505 and 506 were set nearly the same to conceal the influences observed at the right and left ends of the picture element electrode, thereby preventing the contrast from deteriorating.
FIG. 5 shows the layout of picture elements in conventional aligned arrangement. A transistor 603 is formed at the intersection of a signal line 601 and a gate line 602, the transistor 603 being connected to the signal line 601 through a contact 604. One electrode of the transistor 603 is connected to an picture element electrode 606 via a through hole 605. The deterioration in the contrast caused by the disclination is prevented by shielding the area except an opening 607 against light. In the drawing, the opening 607 is positioned so that it is vertically asymmetrical and laterally symmetrical.
FIG. 6 shows the layout of the picture elements in a conventional delta arrangement. A transistor 703 is formed at the intersection of a signal line 701 and a gate line 702, the transistor 703 being connected to the signal line 701 through a contact 704. One electrode of the transistor 703 is connected to an picture element electrode 706 via a through hole 705. The deterioration in the contrast caused by the disclination is prevented by shielding the area except an opening 707 against light. In the layout of the panel having the delta arrangement, picture element cells 708 and 709 employ the same but reversed layouts in order to prevent a flicker resulting from the different characteristic of picture element of each stage and also to prevent the deterioration in the image quality of a stripe pattern or the like. The opening 707 shown in this drawing is vertically asymmetrical but laterally symmetrical.
An attempt to achieve higher definition and reduced size of a liquid crystal display device would inevitably reduce the size of a picture element and the percentage of the area occupied by the opening would reduce markedly if the lightshielding section is made larger to prevent the disclination. For instance, in the case of a picture element electrode of 20 .mu.m square, if the disclination takes place in an area of about 5 .mu.m from an end of the picture element electrode, then the percentage of the area occupied by the opening will be 10 .mu.m square, namely, 25% of the size of the picture element electrode.
The percentage of the area occupied by the opening would be infinitely close to zero percent if the liquid crystal display device is designed to avoid the disclination and the size of the picture element electrode is set to 10 .mu.m square in an attempt to reduce the area of a picture element so as to achieve even higher definition or further reduced size. Reducing the percentage of the area occupied by the opening results in a liquid crystal display device which would let little light to pass through and therefore provide an extremely dark display.
Thus, there was a problem in that fabricating a liquid crystal display device which has a smaller-area picture element unavoidably involves an undesirably small percentage of the area occupied by the opening. This means a dark display image; therefore, in the case of a transmitting-type liquid crystal display device, a bright backlight must be used to obtain sufficient brightness, presenting a problem of increased consumption of electric power.